@DoubleAA thnx, but looking more for a stand-alone reason. not because it's a magic number which appears all over
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rayDec 18 '13 at 19:28

Simplest: Pesach is historical (Exodus to Sea was seven days). Sukkos parallels. Another reason: So that every day of the week is included. Deeper reason: Holidays correspond to aspects of the world (harvest, gathering), 7 is a worldly number (days of the week). Shavuos and Shmini Atzeres are the add on day representing the 8th level, like the days of the Bris - higher than the natural order.
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YishaiDec 18 '13 at 19:36

the talmud says somewhere that the festivals were given for the study of torah. hence it is a kind of sabbatical for learning.
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m.r.Dec 20 '13 at 6:16

The Rambam in Moreh Nevukhim 3:43 says that Pesach is 7 days because a seven day period is an intermediate period between the natural day and lunar month. Just as this has significance in nature (according to Munk, this refers to the medical view at the time that 7 days was a turning point in illnesses), it is significant for the Torah, because the Torah corresponds to and perfects nature. Here is the Schwartz translation:

“As that great revelation took place only on one day, so we keep its anniversary only one day; but if the eating of unleavened bread on Passover were only commanded for one day, we would not have noticed it, and its object would not have been manifest. For it frequently happens that we take the same kind of food for two or three days. But by continuing for a whole period of seven days to eat unleavened bread, its object becomes clear and evident.” -Moreh Nevuchim Chapter 43 – Page 353 in Dover Edition.

Essentially by having the Matzah for a duration of Seven days one might have a greater chance of focusing on it and considering the ideas that it is designed to teach us.